Google has finally integrated storage between its three most data-intensive consumer platforms: Gmail, Google Drive, and Google+. The result? 5 extra GB of total Gmail storage, and 15GB of shared space among the three. This means you're no longer limited to 5GB of Google+ / Drive space, but all three services will now share the same "bucket."
For hardcore Gmail addicts, this comes with an added bonus: all Google Drive storage tiers now apply to your Gmail storage cap. That means a 100GB plan now means 100GB of potential Gmail storage, whereas Gmail had previously been capped at 25GB total storage with the old plans (some legacy plans may have had more than 25GB of Gmail storage).
The changes to storage are rolling out slowly, and may not be yet reflected on your storage settings page, or your Gmail inbox. The updated storage page on your Google account settings will also show how much data each of your Google services is using respective of your total, which is nice.
Google Apps accounts will be getting the shared storage upgrade, too, though they'll be getting a hefty 30GB of combined space.